APL is a write-only language that is optimized for fast matrix and other mathematical operations with a very small number of keystrokes. For example, the program in the subject returns all primes numbers ≤ R. To do this, it uses an extended character set along with a large base of nestable built-ins. This is one of the simpler APL keyboard layouts:

Listening to me discover APL sparked interesting discussions in the 4 cubes that form 'The Grand Ballroom' at work (bad things happen when you let engineers near printers). One of the older guys came over with a sad, wistful look on his face and told me about all the time that he spent trying to read APL code he'd written hours before. He also made the good point that, though it was very fast for its time, it probably doesn't hold up to modern languages that aren't as limited by how much RAM the compiler can eat.

While we were on the topic of write-only languages, someone mentioned TECO, the horribly obtuse language Emacs was first written in. Instant holy war!